This invention relates in general to motor starting devices and in particular to high efficiency saturating reactors for starting three phase motors. The problem of high in-rush starting currents for motors has been long recognized in the art. A number of techniques have been developed for reducing these high motor starting currents, which may be as much as six times the full load motor operating current. A common technique is to insert a magnetic reactor between the AC source and the motor during start up, with the reactor being switched out of the circuit when the motor has achieved a certain percentage of its rated speed. The reactor presents an impedance and thereby restricts the magnitude of current flow to the motor. The amount of the current restriction is related to the voltage drop across the reactor, which is referred to as the back EMF. A most important criterion in a motor starting system is the load imposed upon the power supply. Previously the most efficient starting systems, i.e. those having minimum power (KW) consumption, utilized primary autotransformers which produced relatively low power loss while developing a significant voltage drop for reducing the motor starting current. Primary reactors are less costly than autotransformers but are less efficient.
A large number of motor systems comprise engine driven generators (gen-sets) for supplying the power. For gen-sets, the electrical power efficiency of the motor starting method is of even greater importance since it directly affects the size of the gen-set required.
While greater efficiency and lower cost are always desirable in a starting system, the gen-set is not detrimentally impacted by the higher kilovolt ampere (KVA) or kilovolt amperereactive (KVAR) of motor starting reactors.
It is present practice to provide starting reactors that are specifically designed for a particular motor voltage and current rating. The result is that a supplier of motor starting reactors must stock a very large number of different sizes of reactors, which adds greatly to his costs and increases storage space requirements. Also, present day saturating reactors are inefficient and consume a significant amount of real power (watts) as compared with the desired apparent or reactive power. The back EMF of the reactor is a function of current and whether the current generates real power is immaterial in reducing the starting voltage. Ideally the saturating reactor will have a zero power factor and consume no real power. This would allow a smaller gen-set to be used in a given motor starting application.
All reactor designs utilize tapped electrical coils coupled to a magnetic core. A typical three phase motor starting reactor includes three separate coils, each wound about one leg of a core having three legs and two windows. Each coil has a number of individual taps to provide different impedance combinations for a variety of different starting characteristics.
One reason for the inefficiency of prior art motor starting saturating reactors is the presence of air gaps in the magnetic path in the core structure. The gaps are the result of the type of construction employed where the magnetic core is "pieced" together and welded. Such construction also has a high degree of radiated magnetic flux and required special mounting precautions.
The reactor of the invention is more efficient than those of the prior art and is designed to be driven into deep saturation very rapidly. This provides a low power factor, i.e. minimum coil loss and overall power loss. The reactor of the invention also has a core constructed of conventional thin, insulated E and I shaped pieces of laminated magnetic material that are interleaved to minimize the effects of the small discontinuities between the legs of the abutting E and I pieces. This construction also minimizes radiated flux and permits stacking of units with minimum magnetic interaction between them. The coils are insulated for use with 600 volt motors and are of electrical grade copper in a close wound configuration to efficiently fill the window areas of the core. The conventional 80 percent tap on each coil is replaced with other taps which gives more flexibility in adapting the coils to multiple uses. The construction is such that reactor-to-reactor uniformity is maintained, which permits a supplier to stock a significantly smaller number of reactors. Such uniform reactors may be readily combined (in series and in parallel) to provide a wide range of different motor starting characteristics. Another advantage of the inventive design is achieved by securing the core structure and identical frame members with insulated bolts for minimizing core loss and providing flexibility and ease in installation. With the inventive designs, the number of reactors required by the assignee of the present invention, was reduced from 69 to 6. These 6 basic reactor sizes serve a gamut of 3 phase motors ranging from 13 Hp (20 KVa) through 400 Hp (574 KVa).